Fixes#15653 — Add comment which explains that how a responsive image should be centered
Redo of #15657 which I couldn't get to rebase properly after all the merges were done last night.
People normally center images using text-align. However, the `.img-responsive`
class changes the display type to block. As a result you can no longer align the
image with text-align. You must instead do it with margin: auto. There was no
note about this on the web page, and no mention about setting the display to
block. Users were left on a frustrating journey to figure out why they can't center
images anymore!
